Mugen, from Samurai Champloo!!!
Granted, there are probably more violent characters out there (Lucy from Elfen Lied probably wins this) but he's the most violent anime character that I actually *like*. Check out his fighting style:
It's influenced by Capoeira, a Brazilian dance/fighting technique.
The animation is really incredible in all of the fight scenes he's in. And to be honest... Mugen is kind of a jerk. But he's *my* jerk.